1. Field of the Invention
The present invention is broadly concerned with improved synthetic resin insulators which present continuously renewable hydrophobic dielectric surfaces as a result of normal erosion thereof. More particularly, it is concerned with such insulators which include a continuous phase formed of a synthetic resin such as epoxy, urethane, or silicone rubber, with a discrete, discontinuous hydrophobic phase present in the continuous resin phase as discrete, suspended droplets of hydrophobic material. In preferred forms, the hydrophobic material employed is petrolatum, and additional advantages are obtained when a UV absorber is predissolved or predispersed in the petrolatum prior to incorporation into the continuous synthetic resin phase.
2. Description of the Prior Art
Elongated insulators of various configurations are a staple part of electrical transmission and distribution systems. Although porcelain has long been the material of choice for high voltage outdoor insulators, considerable research has been conducted in the past to develop acceptable synthetic resin insulators. In general, prior resin insulators have proven to be deficient in one or more important areas such as arc track resistance, weatherability, impact strength or castability. Indeed, one of the most demanding applications of polymeric resin materials is in the context of outdoor high voltage insulators.
The most popular polymer resin insulator materials are the epoxies, silicone rubbers, urethanes, and ethylene propylene diene modified rubbers (EPDM). All of these materials can perform well in the field when properly formulated. However, in order to provide an adequate degree of arcing resistance, aluminum trihydrate (ATH) is often used in these resin formulations. As natural erosion of the insulators occur, ATH is continually present on the insulator surface, thereby providing the continuing degree of arc resistance. A drawback of ATH use, however, is the fact that the material is hydrophilic, and the surfaces of insulators including ATH tend to wet out as soon as the initial resin-rich surface erodes away due to weathering or electrical stress. This characteristic wetting out permits the buildup of contaminants on the insulator surface, thus lowering the dielectric capability of the insulator.
In response to this wetting out problem, it has been known to incorporate silicone oil as an additive into resin matrices formed of epoxy for example. Silicone oil is naturally hydrophobic and assists in maintaining surface hydrophobicity on an insulator even after erosion due to weather and/or arcing activity. However, silicone oil stays liquified over a large temperature range. Thus, when the insulator surface erodes and exposes silicone oil, the liquid state of the oil tends to attract dust and encapsulate it. Eventually the silicone oil will become saturated with dust and arcing activity will increase. Moreover, the liquid silicone tends to roll like a bead across a wetted insulator surface and fall to the ground, thereby failing to re-establish the hydrophobicity of the insulator surface.
Many polymer systems can be effectively stabilized for resistance to sunlight by the addition of UV absorbers into the resin matrix. However, certain polymer systems, and especially the epoxies, are difficult to stabilize in this way. Epoxies are such strong UV absorbers that conventional UV stabilizers are not very effective. Accordingly, a prime deficiency of prior epoxy insulators has been their degradation due to UV absorption.
U.S. Pat. Nos. 4,206,066 and 3,838,055 describe prior polymeric insulators. In addition, an article entitled "Molded Electrical Conductors" by Spenadel, et al. and appearing in the August, 1974 issue of Rubber Age at pages 26-33 describes EPDM insulators including plasticizers such as petrolatum, oil and wax. In such applications, the hydrocarbon-based plasticizers are compatible with aliphatic-type resins like EPDM, and therefore are effectively dissolved or dispersed within the resin and do not form discrete, migratible droplets.